This invention relates to a waveguide switch. The advent of PIN diodes in the last decade or so has provided the important additional facility of rapid, low loss switching at microwave frequencies. Typically, the R.F. impedance of a diode can be switched over a range exceeding 10,000:1 (10,000.phi.-0.5.phi.) by changing from a reverse bias of one or two volts to a forward bias current of 0.1 amp. This characteristic has been exploited at microwaves in coaxial line and microstrip, in which the small cross-sectional dimensions of the center, or above group, conductor permit the terminal impedance properties of the device to be realized. In waveguide the problem is fundamentally much more difficult and necessitates a different approach to the problem.